The present invention is in the field of electrical power system control and, more particularly, control for Variable Speed Constant Frequency (VSCF) Electric Power Systems (EPS).
In aircraft electrical power systems, power may be generated by electric machines that may be driven by prime movers such as aircraft engines. These prime movers may rotate at varying speeds and may cause their respective electric machines to produce electrical power at varying frequency. Variable frequency power generation (e.g., 360 to 800 Hertz (Hz)), may not be directly useful for supplying power to frequency sensitive loads.
Various techniques may be employed to produce a substantially constant frequency in these above-described aircraft generating systems. Such techniques may include Integrated Drive Generators (IDG) to maintain the speed constant; two-stage power conversion systems (AC-DC-AC) for frequency conversion to a constant 400 Hz where a transformer-coupled passive rectifiers or more advanced systems such as 12, 18, 24 pulse autotransformer rectifier Unit (ATRU) are used for the front-end AC-DC conversion. IDG subsystems are heavy, expensive and due to their complex high-speed rotating mechanical structure, require frequent costly maintenance. The alternative solution based on a transformer-coupled passive front-end rectifier, is also bulky, heavy, expensive and due to two stages of power conversion, the efficiency is negatively impacted.
In an effort to reduce weight and expense as well as decrease maintenance requirements, various matrix converters along with pulse-width modulation (PWM)-based control algorithms have been proposed in the prior-art. Prior-art matrix converter power topologies may include nine bi-directional alternating current (AC) switches used to connect input AC-system phases directly to a three-phase load. Switching of these bi-directional AC switches may be then PWM modulated to produce the desired output voltage and frequency, as required by the load.
These prior-art matrix converters may offer many theoretical advantages such as the ability to regenerate energy back to the utility, sinusoidal input and output currents, controllable input current displacement factor and an overall reduced size. However, numerous severe technical limitations and failure modes have prevented any practical use of these conventional matrix converters in the aerospace and general industry. Some of these limitations include a) Commutation failures at turn-on and turn-off of switching devices resulting in various component and system level failure modes; b) Due to presence of lay-out inductances in the power-path, significant over-voltages will appear across the switching devices. Conventional methods of passive snubbers or voltage clamps will significantly slow-down the switching times and result in excessive losses; c) direct-coupling of input/output AC system normal and abnormal transients from source to load and vice versa, resulting in significant overstress of key components of the system which may result in their failure; d) Complex and unreliable control algorithm and gating patterns fail to ensure safe and reliable operation under all modes of operation—particularly, during fault modes where software-based mitigation of commutation process is not possible; e) Severe resonance problems associated with input and/or output filters depending on the power topology and configuration; and f) Excessive losses due to selection of AC-Switches which requires significant dead-time, resulting in significant power flow to the active-clamps. These losses result in poor efficiency.
Particularly, the following references show problems and propose complex mitigating methods. 1. Jochen Mahlein, Mark Nils Muenzer, and Manfred Bruckmann, “Modern Solutions for Industrial Matrix-Converter Applications”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 49, NO. 2, APRIL 2002 401. 2. Jochen Mahlein, Jens Igney, Jörg Weigold, Michael Braun, and Olaf Simon, “Matrix Converter Commutation Strategies With and Without Explicit Input Voltage Sign Measurement”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 49, NO. 2, APRIL 2002 407. 3. P. Nielsen, F. Blaabjerg, J. K. Pedersen, “New Protection Issues of a Matrix Converter—Design Considerations for Adjustable Speed Drives”. IEEE Trans. on Industry Applications, Vol. 35, no. 5. pp 1150-1161, 1999. 4. Ziegler, M. Hofmann, W., “Implementation of a two steps commutated matrix converter”, Power Electronics Specialists Conference, 1999. PESC 99. 30th Annual IEEE, August 1999, Vol. 1, pp 175-180. 5. Alesina, A.; Venturini, M. G. B., “Analysis and design of optimum-amplitude nine-switch direct AC-AC converters”, Power Electronics, IEEE Transactions on, Volume 4, Issue 1, January 1989 Page(s):101-112. 6. Huber, L.; Borojevic, D., “Space vector modulated three-phase to three-phase matrix converter with input power factor correction”, Industry Applications, IEEE Transactions on Volume 31, Issue 6, November-December 1995 Page(s):1234-1246.
As can be seen, there is a need to provide advanced matrix converters to overcome these problems so that they may be used safely, reliably and efficiently for various applications in aircraft power systems and general industry.